The compositions of the HR 8799 planets reflect accretion of both solids and metal-enriched gas

With four giant planets (m∼5−10 MJup, Teff ∼900−1200 K) orbiting between 15-70 au, HR 879930 provides an unparalleled testbed for studying giant planet formation and probing compositional trends across the protoplanetary disk. We present new JWST/NIRSpec IFU observations (2.85−5.3 µm, R ≈2700) that now include the spectrum of HR 8799 b, and higher S/N spectra for HR 8799 c, d, and e compared to that in J.-B. Ruffio & J. W. Xuan et al. (2025). We detect CO, CH4, H2O, H2S, CO2, and for planet b, NH3. We combine the NIRSpec spectra with 1−5µm photometry to perform atmospheric retrievals that account for disequilibrium chemistry and clouds, and allow C/H, O/H, N/H, and S/H to scale independently. While the four planets are similarly enriched in carbon and oxygen (volatiles), with C/H and O/H between 3−5×stellar, we observe a tentative trend of increasing S/H – a tracer of refractory solids – from 2−5×stellar with increasing orbital distance. From HR 8799 b’s NH3 abundance, we estimate N/H = 21.2+16.2 −8.8 ×stellar, suggesting the outer planet accreted significant amounts of N-rich gas. Overall, the elemental abundance patterns we observe are consistent with a picture where planet b formed between the CO snowline and the more-distant N2 snowline, while the inner planets accreted 3×stellar CO enriched disk gas within the CO snowline. The excess volatile mass from pebble drift and evaporation implies an integrated pebble flux of 750 ±200M⊕. The increase in the planets’ S/H with orbital distance implies more solid accretion further out, which is quantitatively compatible with expectations from both pebble and planetesimal accretion (2× Minimum Mass Solar Nebula) paradigms.